T cell exhaustion implications during transplantation

Accepted Manuscript Title: T Cell Exhaustion Implications During Transplantation Authors: Mehdi Shahbazi, Mehdi Soltanzadeh-Yamchi, Mousa Mohammadnia-Afrouzi PII: DOI: Reference:

S0165-2478(18)30201-3 https://doi.org/10.1016/j.imlet.2018.08.003 IMLET 6232

To appear in:

Immunology Letters

Received date: Revised date: Accepted date:

21-4-2018 5-8-2018 16-8-2018

Please cite this article as: Shahbazi M, Soltanzadeh-Yamchi M, Mohammadnia-Afrouzi M, T Cell Exhaustion Implications During Transplantation, Immunology Letters (2018), https://doi.org/10.1016/j.imlet.2018.08.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title: T Cell Exhaustion Implications During Transplantation

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Running title: T Cell Exhaustion in Transplantation

Authors and affiliations:

Mehdi Shahbazi1, Mehdi Soltanzadeh-Yamchi1, Mousa Mohammadnia-Afrouzi1*

1. Department of Immunology, School of Medicine, Babol University of Medical Sciences,

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Babol, I.R.Iran

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Address correspondence to: Dr. Mousa Mohammadnia-Afrouzi; Ph.D, Department of

Babol,

I.R.Iran.

Tel:

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Immunology, School of Medicine, Babol University of Medical Sciences, Ganjafrooz Street, +98-1132192832,

+98-1132192959,

e-mail:

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[email protected]

Fax:

Highlights

Exhaustion of T cell due to prolonged exposure to a high load of foreign antigen is

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commonly seen during aberrant immune responses. Little is known with respect to lymphocyte exhaustion during transplantation and its effect

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on aberrant anti–graft responses

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Harnessing the potential of natural processes of T cell exhaustion may provide a novel strategy to improve the conditions of the transplanted patients.

Abstract 1

Exhaustion of lymphocyte function, particularly T cell exhaustion, due to prolonged exposure to a high load of foreign antigen is commonly seen during chronic viral infection as well as antitumor immune responses. This phenomenon has been associated with a determined molecular

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mechanism and phenotypic manifestations on the cell surface. In spite of investigation of exhaustion, mostly about CD8 responses toward viral infections, recent studies have reported that chronic exposure to antigen may develop exhaustion in CD4+ T cells, B cells, and NK cells. Little is known with respect to lymphocyte exhaustion during transplantation and its effect on aberrant anti–graft responses. Through a same mechanobiology observed during chronic exposure of

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foreign viral antigens, alloantigen persistence mediated by allograft could develop a favorable

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circumstance for exhaustion of T cells responding to allograft. However, to achieve better

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manipulation approaches of this event to reduce the complications during transplantation, we need

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to be armed with a bulk of knowledge with regard to quality and quantity of T cell exhaustion occurring in various allografts, the kinetics of exhaustion development, the impression of

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and immune tolerance.

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immunosuppressive agents on the exhaustion, and the influence of exhaustion on graft survival

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Keywords: Exhaustion; transplantation; allograft; T cell

Introduction

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Exhausted T cell is characterized by a setting of impaired T cell function that occurs during cancers and several chronic infections. This phenomenon is manifested through continues loss of proliferative capacity, interleukin (IL)-2 and tumor necrosis factor (TNF)-α and interferon (IFN)γ production, activity of cytotoxic T lymphocyte (CTL), and apoptosis of T cells [1]. The exact

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mechanobiology of T-cell exhaustion still remains unknown [2, 3]. Overexpression of several inhibitory molecules are occurred on exhausted T cells; most importantly, T-cell immunoglobulin and mucin domain-containing protein 3 (Tim-3), programmed cell death protein 1 (PD-1),

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cytotoxic T lymphocyte-associated protein 4 (CTLA-4), lymphocyte activation gene 3, killer cell lectin-like receptor subfamily G member 1 (KLRG1), B-lymphocyte and T-lymphocyte attenuator, Ectonucleoside triphosphate diphosphohydrolase-1 (NTPDase1, CD39) [4, 5], CD160, and 2B4 (CD244) [6]. Studies have reported that blocking of inhibitory receptors results in re-activation of exhausted T cells [7, 8], and a number of trials has evaluated the potential of these molecules as

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therapeutic approaches for cancer and chronic viral infection treatment [9]. In particular,

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investigations about infections and tumor therapy have concentrated on the comprehension of

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efficient memory response generation and deterring the retrieving of T-cell exhaustion as

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therapeutic tools. Nonetheless, triggering of T-cell exhaustion may cause development of selftolerance and transplant tolerance, hence be a beneficial in this way. Allograft tolerance stems

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from immunomodulatory settings following transplantation that is represented through

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immunologic tolerance against the graft. These immunomodulatory occurrences are importantly

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due to involvement of natural regulatory T cells (nTregs), inducible Tregs (iTregs), clonal contraction, clonal anergy, clonal deletion, and clonal exhaustion [10]. It seems that clonal deletion

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is crucial contributing factor to the development of persistent tolerance [11, 12]. T cell exhaustion results in reduction in memory T cells capacity and, therefore, clonal deletion promotes [13].

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Furthermore, T cell exhaustion has been associated with inefficient memory response [14]. Efficient immunologic memory with longevity and persistent immune tolerance are two major goals of the immune response to a given antigen and appears to be challenging purposes of the researches in infectious/cancer immunity in comparison to autoimmunity/transplantation. Several

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line of literature on T-cell exhaustion has concentrated on the infectious and tumor immunity. However, only in recent years, investigations have been drawn toward the transplantation

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immunology and are going to be discussed in this paper.

Immunobiology of T cell exhaustion

Due to marked analogy in functional and phenotypic characteristics of T cells with perturbed function during long-term infections and tumors, the exhausted phenotype of T cells in such

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settings as well as in transplantation has oftentimes been variably implicated as senescence or

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anergy [15]. Exhausted T cells (Figure 1) are defined through a series of molecular aberrant

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expressions, particularly inhibitory receptors, such as Tim-3, PD-1, CTLA-4, CD39 [4, 5], CD160,

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2B4, KLRG1, B-lymphocyte and T-lymphocyte attenuator (BTLA), and lymphocyte activation gene 3 (LAG-3) [6]. Manipulation of these signaling pathways through these molecules can

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reverses the T cell’s effector dysfunction, proposing that T cell exhaustion is an active event under

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the regulation by inhibitory molecules and not an end stage of differentiated procedure. Although

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no unique transcription factor determining T cell exhaustion phenotype, both exhausted CD8+ and CD4+ T cells demonstrate a distinguished molecular profile from that of effector and memory T

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cells and have a set of common transcription factors like B-lymphocyte-induced maturation protein (Blimp) 1, activating transcription factor (ATF), and basic leucine zipper transcription factor [16].

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Furthermore, it has been stablished that exhaustion of CD8+ T cells may be a stable and heritable stage of differentiation demonstrating the functional adaptation of the T cells to save the host from redundant and pathologic immune responses and in parallel cause a control on virus proliferation [17-20]. T cells, under specific circumstances, lose their proliferative and functional abilities that might be reversible up until some levels. It seems that further studies should be performed to 4

characterize T cell exhaustion more clearly, acknowledging the mechanobiology exerted and recognizing specific molecular and phenotypic manifestations of these cells.

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Mechanisms affecting T cell exhaustion

Chronic exposure to antigen are thought to be crucial contributing factors for the promotion of T cell exhaustion [21-26]. Inoculation of three different doses of lymphocytic choriomeningitis virus (LCMV) in mice has been demonstrated to be leading to diverse disease outcomes [20]. Efficient

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CD8+ T effector cells was developed through low doses of LCMV inocula, resulting in little

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disease manifestations with respect to lung and liver lesions due to virus. On the other side, high

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dose of LCMV led to T cell exhaustion with greater virus lasting, but negligible immunopathology.

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Nonetheless, a moderate dose of LCMV inocula resulted in partial T cell exhaustion but remarkable mortality rate. Survived mice with moderate dose inoculation demonstrated viral

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persistence and a bulk of immunopathologic outcomes, such as liver and lung necrosis. This

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observation implies that T cell’s clonal exhaustion may be protective responses during infections

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with non-cytopathic viruses, such as LCMV, hepatitis B virus (HBV) and hepatitis C virus (HCV), promoting less severe pathologic outcomes and mortality. With respect to transplantation, there is

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an association between the mass the tissue engrafted and the status of rejection or acceptance of the graft. It has been observed that transplantation of two hearts and two kidneys at the same time

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eventuated in long-term acceptance of the graft, while there was acute rejection of allogeneic heart and kidney graft separately [27]. Furthermore, another investigation validated this phenomenon, in which deletion of the alloreactive T cell reservoir led to CD8+ T cell exhaustion, resulting in long-lived liver allograft survival in comparison to cases with transplantation of tissues with small size [28]. Moreover, it was demonstrated that a distinct count of T cell is needed for rejection 5

development exerting skin, islet, and heart transplantation models [29]. Low ratio of T cell number to donor tissue mass is an important element of hypoactivation of responses to engrafted tissue as well as graft survival [30]. This hyporesponsiveness of donor-specific T cells is regulated by the

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PD-1: PD-1 ligand (PD-L1) signaling pathway [31]. Low-level of mismatched grafts was led to spontaneous tolerance [32] and spontaneous acceptance of murine liver allograft is contingent upon PD-1:PD-L1 pathway, implying to critical importance of T-cell exhaustion during spontaneous graft tolerance [33].

In transplantations with comparatively solid mass, like heart engraftment, T cells demonstrated

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diverse abilities in infiltration to the allograft and behaved differently, in which T cells that could

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not migrate to the transplanted tissue revealed characteristics of an exhausted cell [34]. These

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observations are in line with another data, in which recipients with deficiency for chemokine (C–

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C motif) ligand 5 (CCL5-/-) and abnormal T cell recruitment ability could not remove the LCMV

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infection, and CCL5-/- CD8+ T cells presented exhausted phenotype [35]. Considering all of these

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data, it can be concluded that T cells might be instructed at the site of response to become activated, promote effective memory responses, and deter exhausted phenotype. Furthermore, it has been

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demonstrated that T cell incubation with inflammatory cytokines, such as type I interferon (IFN), IL-12, and IL-18, culminates in increased proliferation and intense effector function [36-38]. IL-

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12 reduces the potential of T cells to be exhausted [39] as well as plays a role in reversing exhausted

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T cells [40]. It has been established that several infectious elements, have promoted natural mechanisms to deter or hinder inflammasome activation, which develops efficient immune responses against infectious agents [41, 42], to escape the immune response [43-45], probably by developing pathogen specific exhausted T cell.

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Von Hippel-Lindau-deficient CD8+ T cells have characteristics of enhanced glycolytic metabolism, overproduction of effector cytotoxic mediators like perforin, granzyme B, IFN-γ and TNF-α, and present higher potential of controlling of viral infections and tumor growth through

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hypoxia-inducible factor (HIF)-1α–dependent approach [46]. Therefore, manipulation of the HIF pathway in T cells to activate them by suppressing von Hippel–Lindau function leads to bypassing the exhaustion process and maintaining the effector functions of CD8+ T cells. In accordance with this observation, it was shown that expression of HIF-1α in infiltrating inflammatory cells to the renal allograft tissues associated with the level of rejection, impaired function of transplanted

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tissue and poor allograft survival [47]. These information offers that hypoxia at the tumor and graft

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microenvironment as well as the site of infection may modulate the T cell differentiation and

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responses by means of the HIF pathway [48].

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Impaired CD4+ T cell contribution has been observed to be accompanied with the intense CD8+

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T cell exhaustion. Moreover, adoptive transferring of CD4+ T cells to restore their help leads to

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improved virus specific responses [49]. Additionally, IL-2 has been reported to have synergistic effects with PD-L1 blockade in improved function of exhausted T cells [50]. On the other side,

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removal of regulatory T (Treg) cell in chronically infected mice culminated in a marked reinvigorated function of virus-specific exhausted CD8+ T cells. Treg cell depletion alongside

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with PD-1 pathway blockade resulted in efficient viral control. As a consequence, Treg cells are involved in CD8+ T cell exhaustion maintenance during chronic infections [51, 52]. Additionally,

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activation of CTLs through Treg-conditioned CD80/86lo dendritic cells (DCs) in the tumor microenvironment developed an upregulation of Tim-3 and PD-1 molecules, leading to tumorinfiltrating CTL dysfunction [53]. It appears that the tissue microenvironment exerts an important

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role in controlling the T cell exhaustion through antigen presentation to the T cells within a hypoxic and inflammatory context.

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Induction of T cell exhaustion to manipulate immune-based disorders

CD8+ T cell exhaustion demonstrates plasticity that can be reversed, hence induction of T cell exhaustion could be considered as a promising therapeutic tool for immune-based complications with increased immune system responses. This is in line with the observations in which exhaustion

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settings, presented by transcriptional profile, correlated with amelioration of relapsing course in

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autoimmune diseases [54]. Furthermore, it was reported that infection by LCMV-clone 13 can

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inhibit the virally induced autoimmunity in animal models expressing viral autoantigens [55]. As

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such, autoimmune disease development was accelerated in mice with deficiency in inhibitory receptors [56-58]. Alternately, during checkpoint therapy of cancers, the rate of autoimmune

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development was revealed to be high [59]. The potential of reversing the CD8+ T cell exhaustion

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in therapy was evaluated through CD8+ T cell co-stimulation using antibody. It was demonstrated

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that CD2 signaling could prevent the development of exhaustion phenotype in both animal models with LCMV infection and in humans with autoimmune disorders [54]. It this way, CD2 stimulates

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the expansion of IL7RhiPD1lo colon resulting in improved cell survival and T cell:antigen presenting cell (APC) adhesion, enhancing peptide/major histocompatibility complex (MHC):T

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cell receptor (TCR) signaling sustenance by binding to CD58/LFA-3 [60], culminating in alteration in stimulation-induced apoptosis settings [61] as well as the IL-2 signaling [62]. Enhanced power of peptide/MHC:TCR signal causes promoted survival of expanding T cells [63], although the effect of CD2 has been postulated to be moderate as it was observed that CD2-/- mice demonstrated a normal immune system with pristine primary and memory responses of T cell [64]. 8

Nonetheless, these observations from murine models need to be interpreted generally with awareness, since there is no a murine analogy of the high-affinity human CD2 ligand, namely CD58/LFA-3 [65]. Addition of an Fc-chimeric PDL-1 blockade can confine the differentiation

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process of a vigorous IL7RhiPD1lo colon [54], implying to the potential of checkpoint agonists in deterring the T cell response toward an exhaustive phenotype. This process should be precisely examined in vivo, and also the modulation of exhaustion, which is accompanied by clinical outcome in human disorders, needs to be evaluated for improving the course of relapsing diseases. Genetic polymorphisms in the CD2 signaling pathway have been associated with several

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autoimmune disorders [66, 67] and have been manipulated with promising positive outcomes in

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patients with type 1 diabetes (T1D) [68] and psoriasis [69], in which T cells were depleted by

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exerting CD2 blockade instead of signaling pathway modulation. It is still unknown if CD2-

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mediated costimulation inhibition, which has already been usual in treatment of autoimmune

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complications, might impress the development of exhausted T cell. It has been reported that

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differentiation of CD8+ T cells to memory T cells is mediated through a short span but strong stimulation, and after it is received the differentiation process continues without requiring further

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stimulation and signal receiving [70]. Hence, inhibiting the costimulation process leads to interruption in memory responses. Nonetheless, costimulation blockade might not act in the same

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way on reversing the exhaustion process, as it might not be continued after initial signaling

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delivery.

T cell exhaustion as a therapeutic approach in transplantation

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Leukocyte migration inhibitory factors have been approved for treatment of multiple sclerosis (MS) [71, 72]; as well, others are in the process of examination for potential clinical application for other inflammatory settings. That notwithstanding, the final condition of migratory inhibited

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autoreactive T cells in such inflammatory settings has not clearly been characterized. Increased rate of progressive leukoencephalopathy, a condition that associates with chronic John Cunningham (JC) virus infection, with the application of migration inhibitory factors [73] may stem from contribution of virus-specific exhausted T cell, implying to the role of leukocyte migration inhibitory agents in the development of T cell exhaustion. Additionally, CCL5-/-

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recipients with dysfunction in T cell recruitment mechanisms could not clear LCMV infection and

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CCL5-/- CD8+ T cells showed exhausted presentations [35]. With respect to transplantation, the

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microenvironment is hypoxic and seems to be the major characteristic of inflammasome activation

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after ischemia-reperfusion harm. As a result, the graft microenvironment seems to develop both known contributing factors for hindering T cell exhaustion, namely inflammatory cytokine

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signaling and HIF stabilization. Hence, it is hypothesized that if T cells are inhibited from

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approaching the inflammatory microenvironment of the graft, they might develop exhausted

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phenotype due to missing additionally signals from the target engrafted tissue. In line with this conclusion, it has been observed that fucosyltransferase-VII-deficient (Fut7-/-) recipients having

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impaired recruitment of leukocytes due to dysfunctional selectin molecules demonstrated longlived graft with less vasculopathy as well as manifestations of exhausted CD4+ T cell [35].

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Considering together, it is suggested that exhausted T cell exerts a beneficial role in long-term allograft survival and that manipulating the leukocyte recruitment pathway seems to be a novel mechanism of exhausting in CD4+ T cells. It would be promising tool for induction of T cell exhaustion by inhibiting the leukocyte recruitment and, therefore, develop immune tolerance and

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interrupt transplant rejection. Investigations in the context of reducing transplantation complications should primarily concentrate on promoting Tregs (possibly through IL-2 pathway)

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as well as interrupting T cell contribution (via blocking of cytokines and costimulations).

T cell exhaustion and transplantation

Despite extensive exploration of T-cell exhaustion mechanobiology in cancers and chronic viral infections, a little number of literature has focused on this event in transplantation [74, 75]. T cell

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exhaustion has probably been implied as donor-specific hyporesponsiveness in different studies

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[76, 77]. Several inhibitory pathways, including Tim-3:galectin-9, CTLA-4:CD80/86, and PD-1:

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PD-L1/2 has been reported to be upregulated in exhausted T cells and exert an important role in

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autoimmunity as well as in transplant tolerance [78-83]. CTLA-4 is overexpressed on exhausted T cells, particularly on exhausted CD4+ T cells [84], and evidence implies that CTLA-4 signaling

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is needed for development of transplant tolerance [82, 85]. As such, Tim-3 signaling is involved

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in modulation of alloimmune responses [86-88]. PD-1 pathway, also involved in T-cell exhaustion,

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participates in transplant tolerance as well as autoimmunity [78-82, 89]. Taking these data together, T cell exhaustion may be an important mechanisms of maintenance of allograft tolerance.

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T cell exhaustion in engraftment process has been directly observed in a study [28]. An immediate and wide spectrum activation and proliferation of host CD8+ T cells against allograft in the initial

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period after liver transplantation was indicated that was followed by exhaustion of CD8+ T cells, which did not respond toward alloreactive T cells in vitro. Furthermore, in a model of chronic allograft rejection T-cell exhaustion was observed, as Fut7-/- recipients revealed long-lived graft survival with little vasculopathy complications in comparison to controls [34]. It was detected that

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allograft survival was associated with exhaustion of CD4+ T cell in the periphery, manifested through impaired proliferation, upregulation of inhibitory molecules like PD-1, Tim-3 and KLRG1, defective cytokine production, downmodulation of IL-7Ra on CD4+ T cells, and declined

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number of CD4+ memory T cells within the allograft. On the other side, PD-1 blockade caused a reverse procedure in CD4+ T cell exhaustion, while removal of Tregs from donor indicated beneficial effects neither in wild-type or Fut7-/- hosts [34]. These evidence also imply strongly to the important role of T cell exhaustion in developing transplanted tissue survival as well as transplant tolerance.

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Treg cells have been associated with long survival of graft in a 5 year follow up of cases with

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kidney recipients alongside with the infusion of donor bone marrow cells (DBMC), which is a

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microchimerism-based approach to decrease the intensity of immune responses toward the donor

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engrafted organ [90]. This approach was related to decreased levels of inflammatory and

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proinflammatory responses manifested by declined numbers of IFN-γ and IL-17-producing T cells,

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while an increased number of IL-10 producing cells was observed [91]. Additionally, transplanted cases who also received DBMC indicated increased serum levels of TGF-β1, which is also act

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similar to IL-10 in suppressing the immune responses [92]. Treg cells regulate the immune responses through producing cytokines like IL-10 and TGF-β1 [93]. As a result, these cells may

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be beneficial in synergizing the other immune-controlling approaches, such as DBMC infusion

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and exhausting T cells.

T cell exhaustion during hematopoietic cell transplantation and graft-versus-host disease

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There is evidence demonstrating the restoration of immune system after transplantation, which occurs alongside of exhaustion. Immune recovery after non-myeloablative transplantation was observed to mainly originate from peripheral expansion of the graft-contained mature T cells

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during the early months, whereas neo-generation of T-cell by the thymus led to immune reconstitution during a long time period [94]. Furthermore, T cell neoproduction after allogeneic hematopoietic cell transplantation (HCT), at least in cases under 60 years old, was detected to be minimal and reconstruction of T cell immunity depended highly on intense peripheral expansion of T cells within in the graft. Function of thymus was under the impression of chronic graft-versus-

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host disease (GVHD) [95]. Moreover, evidenced demonstrated that early after HCT, temporary

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depletion of donor CD4+ T cells effectively prevented GVHD. However, it maintained strong

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graft-versus-leukemia (GVL) impressions in allogeneic and xenogeneic murine models of GVHD.

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On the other hand, the interactions of PD-L1 with PD-1 on donor CD8+ T cells led to anergy, exhaustion, and apoptosis, which in turn prevented GVHD [96]. Although in early stages of

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transplantation, there is an exhaustion of T cells, there might be immune recovery later, originating

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from peripheral expansion of T cells and GVHD play a role in this process by affecting thymus.

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During hematologic malignancies, allogeneic stem cell transplantation (allo-SCT) can develop alloreactive T cells that recognize minor histocompatibility antigens (MiHA). However, escaping

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mechanisms exerted by tumor, through low expression of costimulatory molecules and can upregulate co-inhibitory molecules, can cause failure of T cell immunity. A study reported that

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MiHA-reactive CD8+ T cells expressed CD27++/CD28++ phenotype and KLRG-1 and CD57 expression was downmodulated. There was also upregulation of inhibitory/exhausting molecules, including PD-1, TIM-3, and TIGIT on CD8+ T cells. PD-1 and TIGIT were seen to be involved in regulation of T cell-mediated tumor control and relapse of patients after allo-SCT [97].

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Therefore, PD-1 and TIGIT are involved in regulation of tumor mediated by T, suggesting that antibodies against these molecules could be promising in therapy of allo-SCT followed by a

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relapse.

The challenges in the way of T cell exhaustion induction in therapeutic strategies

Therapeutic approaches with respect to induction of T cell exhaustion to increase survival of grafts seems to be a logical hypothesis. Nonetheless, several important challenges and caveats are in the

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way, requiring further considerations. First of all, reversing the CD8+ T cell exhaustion is mediated

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through sustained overexpression of the inhibitory receptors that promotes the exhaustion process.

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Indeed, distinct surface molecules have been identified that are subject to be blocked. A checkpoint

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agonist, for promoting the exhaustion efficiently, should bind strongly to the co-stimulated CD8+ T cells, which express only low levels of inhibitory receptors on themselves. This issue might

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confer potential difficulties for therapeutic induction of T cell exhaustion. Nonetheless, inhibitory

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receptors, by all means, might not be overexpressed on functionally exhausted cells, which is

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observed in the models with chronic viral infections of LCMV. Up to now, a little number of investigations has provided data with the molecular mechanobiology of exhaustion in the context

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of transplantation; however, it seems that different combinations of inhibitory molecules in various diseases casus T cell exhaustion [54]. Therefore, it may be plausible to develop exhaustion in the

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right way through choosing suitable molecule or combination of molecules according to the specific disorder in order to ameliorate disease complications. At the second level, it is important to consider the timing of application of further inhibitory signals. It should be noted that early PD-1 signals exert an inhibitory function of exhaustion

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process instead of promoting it. Furthermore, persistent rather than transient signals of early PD1 play role in efficient triggering of exhaustion [98]. As a result, therapeutic approaches with applying exhaustion in T cells should be able to provide long-lasting influences to maintain

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exhaustion induction therapeutically and to ameliorate disease complications. Studies have demonstrated that chronic viral infections modify the epigenetic mark of PD-1 locus by demethylating its CpG sites, which seems promising therapeutic mechanism as they cause persistent effects [99]. Nowadays, it has not been investigated if inhibitory receptor blockade or agonist may confer sustained induction as epigenetic modification can do [100].

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Third, inducing exhaustion may result in adverse events. The purpose of therapeutic induction of

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exhaustion is to modify the mechanobiology of long-lasting clinical manifestations in

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transplantation settings. This foal need to be achieved without eventuating in predisposition to

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increased infectious events or tumors, which both are typically observed problems alongside with

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the application immunosuppressive therapy. Combinatory blockade of inhibitory receptor can

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produce synergistic reversal of exhaustion procedure [101, 102]. As a result, similar combinations of agonist signals may culminate in analogues level of exhaustion to decline graft rejection

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progression and perpetuation while minimizing unwanted events.

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To pave the way toward optimal development of therapies by modulating the T cell exhaustion, we need to armed with better understanding of the underlying Immunobiology of this process.

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Considering the observation that only a group of cancer subjects demonstrated long-lived responses to checkpoint blockade, a number of studies have initiated to explore the predictive biomarkers underlying such responses with improved efficacy and minimized adverse effects [103]. To come up with better and efficient exhaustion therapy in the heterogeneous context of

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transplantation settings, it would be favorable to identify biomarkers that cause hierarchical development and application of such treatments.

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Concluding points

Nowadays, studies are on the way to precisely identify the role of T cell exhaustion in interrupting the allograft rejection and developing the transplant tolerance process. Exhausted T cells present overexpression of various transcription factors and inhibitory molecules mediating their impaired

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functions. Currently, several clinical trials are being conducted, on the other hand, to develop

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agents against inhibitory pathways for reverting the exhaustion of T cells for cancer therapy as

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well as treating chronic infections [104-106]. Nonetheless, a number of reports has established the

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unwanted side effects associated with immune hyperresponsiveness and exacerbation of autoimmunity [82, 106], leading to concerns about application of this mechanism in allograft

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rejection. It is challenging to develop mechanisms that trigger the inhibitory pathways to cause

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exhaustion in T cells for decreasing the immune adverse responses toward self or transplanted

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tissues. However, limiting effects of CD4+ T cells, including transplant microenvironment modulations or confining the access of T cells to the allograft microenvironment, appear to be

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promising tools to develop exhaustion in T cells to interrupt transplant rejection and promote transplant tolerance. Along with promotion of T cell exhaustion, other immunosuppressive

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approaches like exertion of Tregs is better to be considered. Last but not least, harnessing and acknowledging the potential of natural processes of T cell exhaustion may provide a novel strategy to improve the conditions of the transplanted patients.

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Acknowledgement We wish to thank the Immunology Department Facility at the Babol University of Medical

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Sciences.

Disclosure of conflict of interests

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None.

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References

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Figure 1. Naïve T cell development and differentiation pathway. Upon antigen recognition, naive T cells differentiate into functional effector T cells in case of inflammatory conditions and if

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enough amount of costimulatory signaling is provided. On the other side, naive T cells differentiate into dysfunctional unresponsive T cells, if the conditions tend to be tolerogenic. In case of persistent antigen stimulation because of high antigenic load (such as transplanted tissue or allograft) and sustained inflammation, effector T cells gradually lose their potential to expand and produce cytokines and overexpress several inhibitory receptors, such as Tim-3, CTLA-4, and PD-

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1, which transduce downstream signals to trigger exhausted phenotype of T cells. These cells,

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initially can be rescued after the inhibitory signaling blockade but finally become end-stage

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differentiated and tend to be apoptosed. However, exhausted T cells can reverse to be effector cells

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if they are persistently stimulated by the same antigen.

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